In thermally stressed gas turbine components, such as for example guide vanes or rotor blades or liners in the combustion chamber, Ni-based superalloys, the surfaces of which are additionally protected by the application of a ceramic thermal barrier coating, are often used in order to reach high operating temperatures and thereby to achieve improved efficiencies. An example of a structure of a component of this type in the form of a turbine blade is illustrated in excerpt form in FIG. 1. A bond coat (BC) 12, for example made of MCrAlY or PtAl, has been applied to the substrate 11 composed of a Ni-base superalloy. The actual thermal barrier coating (TBC) 14 has been applied to the bond coat 12. At high temperatures, a thermally grown oxide layer (TGO) 13 composed predominantly of α-Al2O3 is formed at the interface between thermal barrier coating 14 and bond coat 12, which slows down the oxidation of the bond coat.
The service life of the thermal barrier coating 14 under cyclic oxidation is dependent not only on the thickness of the thermally grown oxide layer 13 (on account of increasing stresses at the interface as the oxidation increases) but also on the strain tolerance of the thermal barrier coating 14. The service life is limited in particular by delamination from the substrate 11.
The mechanical properties of the thermal barrier coating 14 are determined by various mechanisms:
The following are driving forces in the TBC delamination:    A1: transient strains ε1 in the event of thermal shocks (temperature gradients for example when starting up and shutting down a gas turbine) and    A2: stationary mismatch strains ε2 (mismatch between the thermal expansions of adjacent layers for example during steady-state gas turbine operation)    A3: intrinsic strains ε3 caused by TGO growth at the TBC/BC interface (oxidation rate of the bond coat 12)    A4: although further strain components (e.g. mechanical) are present, they can substantially be ignored for the use of coated parts in gas turbines.
The result of the local strains A1-A4 is as follows for the local stress σTBC in the TBC:    A5: σTBC=ETBC Σ εi, where i=1 . . . 4; ETBC: macroscopic modulus of elasticity of the TBC
The driving forces in the TBC delamination are in some cases independent of operating time (component A1, A2, A4) and in some cases dependent on operating time (component A3).
Resistance to TBC Delamination:
According to linear fracture mechanism theory, the thermal barrier coating fails as soon as the energy release rate G reaches a critical value GC, where:
B1: G>=GC=π(σTBC)2 a/ETBC=(KIC)2/ETBC with critical crack length a, fracture toughness of the TBC KIC and σTBC according to A5.
(Source: Anderson, T. L.; Fracture Mechanics; 1994; ISBN 0-8493-4260-0; p. 16)